Inserting a record selection formula at design time using Select Expert

In my report I am using a Stored Procedure as the datasouce that returns me a ShowinList flag and DuplicateSettings information together with other data. If the ShowinList flag is set to '0' I need to show records whose DuplicateSettings is null and to show all records without any filtering if this flag is set to '1'. For this I had written the following formula in the Select Expert:

if {PKG_REPORT_SSP_GET_WORD_REPORT_FULLTEXT.ShowInList} = 0 then
IsNull({PKG_REPORT_SSP_GET_WORD_REPORT_FULLTEXT.DuplicateSettings})

This code works fine if the ShowinList flag is set to '0' but generates a blank report if the ShowinList flag is set to '1' even though there is data in the database.

Could anyone please suggest a solution to this problem.

Thanks